Alumina sintered bodies have been used in various industrial fields, making use of the feature of having excellent properties such as high hardness, high strength, high heat resistance, high wear resistance, and high chemical resistance. As one of such applications of alumina sintered bodies, a grinding wheel is mentioned.
Special alloys are often used as materials for parts constituting transportation equipment typified by automobiles or industrial machines. Since such special alloys are harder than normal SUS304 or the like, heavy-grinding wheels having a high “grinding ratio” which have not existed in the past are required in the market for processing these alloys. Here, the “grinding ratio” is an indicator showing the performance of grinding wheels and is represented by the following formula. A higher grinding ratio indicates higher performance of grinding wheels.Grinding ratio=Amount of work materials ground (grinding amount)/Wear amount of grinding wheel  (A)
Generally, if many work materials can be ground with few grinding wheels, the performance is determined to be good, but the grinding ratio of a grinding wheel is affected by the hardness, the strength, and the toughness of abrasive grains used for the grinding wheel. Further, phenolic resins and the like are mainly used as binders, and therefore the abrasive grains are required also to have resin affinity. The following relationships are considered to exist between the grinding ratio and the hardness, between the grinding ratio and the strength or the toughness, and between the grinding ratio and the resin affinity:    (1) As the hardness of abrasive grains increase, the grinding amount also increases, and therefore the grinding ratio also increases;    (2) As the strength or the toughness increases, the amount of abrasive grains broken decreases, and therefore the grinding ratio increases; and    (3) As the resin affinity of abrasive grains increases, shedding of the abrasive grains from grinding wheels hardly occurs, and therefore the grinding ratio increases.
That is, the numerator in the formula of the grinding ratio is affected by the hardness of abrasive grains, and the denominator therein is affected by the strength or the toughness and the resin affinity of abrasive grains. In order to obtain a grinding wheel with a large grinding ratio, it is ideal to increase all of the hardness, the strength, the toughness, and the resin affinity.
For example, Patent Literature 1 discloses allowing a fine isotropic crystal and a crystal having an anisotropic shape to coexist by mixing a metal oxide having a eutectic point with Al2O3 of 1600° C. or less with Al2O3 powder, followed by firing using microwaves.
Further, Non Patent Literature 1 discloses that CaO and SiO2 are uniformly co-doped in an alumina raw material in order to allow a plate-shaped alumina crystal having a large anisotropy to be contained in an alumina sintered body.
Further, Patent Literature 2 discloses layering an aluminum oxide material containing a crystal growth inhibitor and an aluminum oxide material containing a crystal growth promoter, followed by sintering the obtained laminate in one step.
Further, Patent Literature 3 discloses applying a solution or a slurry containing a magnesium compound to an aluminum oxide compact or a pre-sintered body, followed by main sintering. Patent Literature 3 discloses that a multilayer aluminum oxide sintered body in which the surface layer is composed of an equiaxed crystal with a small grain size, and the inside is composed of an anisotropic crystal can be obtained according to this production method.
Further, Patent Literature 4 discloses a multilayer alumina sintered body having an internal layer and a surface layer that contains silicon on the surface and has the same constituent crystalline phase as the internal layer, with their average crystal grain sizes being different from each other by 30% or more.